Here is a step by step for repairing scratches in gelcoat. This would apply
to deeper and/or wider scratches, where the raw fiberglass is showing
through. However, you can use this same method on small scratches that are
too deep to buff out, but do not go all the way to the raw fiberglass. The
only difference would be that less agressive sanding is required, during the
initional preparation and the process of sucessively overfilling with
several coats of gelcoat will take fewer over coatings (perhaps only one
coat will be required).
With a little practice, you should get very professional looking results,
using these steps.

Here's what you'll need to do this repair.

gelcoat or gelcoat paste
(I prefer plain old gelcoat, because I have found that it doesn't seem to yellow with time
as much as the paste.) If the scratch is really deep, or wide get some
of each

mek hardener

small can of acetone

plastic cup, or mixing board (board is also plastic)

squeegee (the plastic kind for spreading bondo) or a tongue depresser, or a
small plastic putty knife will work, but not as easily.

roll of waxed paper

sand paper (grits below)

hard rubber sanding block

spounge or spounge rubber sanding block

pair of chemical resistant, rubber gloves

roll of masking tape and some paper (news paper or masking paper)

a few clean rags

For sandpaper, you will need a few different grits:

#180 to #220

(dry production type, aluminum oxide works best, which is a very light grey
or whitish on the gritty side. the brown stuff doesn't cut the gelcoat too
well)

#400

#600

#800 to #1200

#1500 to #2000

All of the above, starting with the #400 are wet or dry type.

First, clean the area well. If it is at or near the waterline, particularly.
Be sure there is no oily residue around, or in the scratch. Now check the
damaged area for loose chips of gelcoat along it's edges. I usually do this
by running a fingernail along the edge. You will be able to feel loose chips
and break them off. Go easy..... a sliver of gelcoat jabbed up under your
fingernail doesn't feel very good. ;)

Clean the area very well again with acetone and a clean rag. Turn or fold
the rag a number of times and wipe the area with the clean side, wetted with
acetone. This removes any residual dirt, oil and wax. Clean right down in
the scratch and around it so there is a perfectly clean area of about 2 or
three inches all around it. WEAR THE RUBBER GLOVES while working with the
acetone. It soaks into skin readily and is very hard on your liver.

Now, using the dry sanding paper, sand the scratch well. It helps on
scratches to wrap a little paper around a pencil, pen or other small, round
object. You are sanding just in the scratch itself and along it's edges
here. Don't sand an area around the scratch. Just sand in the scratch itself
and smooth any jagged edges. Once again check for loose chips of gelcoat
along the edges, as you sand. Try to give the scratch a consistant shape and
depth along it's length in this process. The best shape would be a V,
however a cupped shape will work also. Don't deepen or widen the scratch a
lot trying to get this shape however. The gelcoat is quite forgiving.

Using another clean rag, wash out the scratch with acetone again. (remember
the gloves)

Now, (finally) you are ready to start filling the scratch with new gelcoat.
Remember, in gelcoat repair, as in the auto paint and body industry, the
repair will only be as good as the preparation, so follow the above steps
carefully.

If the scratch is small, you may be able, at this point to do one filling of
gelcoat paste and be done with it. If it is deep and/or very wide a few
fillings may be required. If you are using plain gelcoat, you will surely
have to coat the scratch several times, unless it is very small. If it is a
very large and deep scratch you will start filling with the paste and use
gelcoat for the final coat or two, for a very nice repair.

Take your plastic squeegee and trim off a piece about a inch wide (more or
less), so you have a little narrow one. Keep the, probably wider part of the
squeege handi though.

Mask off an area around the scratch, particularly below it, so any drips of
gelcoat will not end up on the good finish. Don't mask too close to the
repair area itself though.

Mix a small amount of paste, or gelcoat in the cup or on the mixing board.
follow the directions carefully. I find in warmer climates, a little less
MEK hardener will keep the gelcoat from kicking too soon.
Apply the gelcoat, or paste with the small squeegee. You are not trying to
completely fill the scratch here. You will repeat this process several
times, until the scratch is filled completely. Let each coat cure to a tacky
feel, before you add another coat.This way you won't have a blob of gelcoat
come dripping out of the scratch. You will then add a final coat that brings
the surface of the repair, just higher than the original finish. We're not
trying to make the repair look perfect here. We are just filling in the
scratch. When you feel the repair is equal to or slightly above the original
surface, cover the final coat of gelcoat with a piece of waxed paper, while
it is still wet. Smooth out the paper with the leftover clean squeegee and
tape it in place. Let things stand as they are for the next 20 minutes, at
least. If you can't get back to it, at this point, until the next day,
that's okay too.

When cured, remove the waxed paper and inspect the repair. There will be
some shrinkage of the gelcoat and it may still be low in places. don't try
to fill these low spots yet.

Take the #400 paper and cut a piece to fit the hard rubber sanding block.
You can have a bucket of water handi to dip the block in as you go, or you
can run a trickle of water from a hose over the area as you sand. You must
use the sanding block in these following steps, to make the repair look
right. Don't sand with your hands and paper alone. The end result will look
wavey. You will start to be able to see where the paper is sanding and
where it isn't, by drying the area off every now and then and looking
closely. The areas being sanded will be dull and the low spots and original
finish around the repair will still be shiney. If you start dulling the
original finish around the repair, at this point, stop sanding and add
another coat of gelcoat. When the area is reasonably fair, but has a few low
spots (you will be able to see them easily now, as they will be little
shiney places in the middle of your sanding field) add another coat of
gelcoat. Sand lightly again with the #400. Keep the water trickling, or keep
dipping the block in the bucket as you go. This step is not the final one,
so don't over sand. The original gelcoat is fairly thick, but it does have
it's limits.

Now, rinse off the repair well, with clean water (not the water in the
bucket). Remove your #400 grit from the block and rinse the block well too.
Install the #600 paper and wet sand the repair with it for awhile. The idea
here is to remove the scratches created by the #400 paper. If there yet
remain some low spots, this will be the final chance to fix them. They
should be indeed small, or non-existent by this time. You will also notice
that the #600 paper will cause the repair to start taking on a dull shine
again.

If, at this point you are happy with the look of the repair (here we're
looking for a flat, smooth look that blends out into the original surface),
you can go to the next step. If the repair is still lumpy, and or uneven, go
back a couple steps. You'll have to scuff the area with the #180 or #220
some if you need to add some more gelcoat. Add a coat and cover with waxed
paper again. Follow steps to this point and continue.

You are now ready to burnish the area with the #800 to #1200 grit paper. You
are still wet sanding and you are still using the sanding block. You are
only trying to remove scratches caused by the last grit used, so this won't
take long at this point. You will see the glow, or shine getting glossyer
when you dry off the repair. BTW Stop and dry the area regularly throughout
the finish sanding process to check your work.

After using the #1200 grit, some will go ahead and buff and wax the repair.
It will actually be easier to go to the next grit and clean up the repair a
little more. But you have to look at the overall finish. If the rest of the
boat looks like glass, take the next step. If not stop here and get the
whole boat buffed out.

If you're moving on, go to the #1500 to #2000 paper. Here you will, perhaps
find that the hard rubber block seems to stick, or drag on the surface. If
so, wrap the sandpaper around the spounge instead and wet sand this way.

You should now have a repair that nobody will ever know about, except you
and me ;). The only thing will be the color match. This, I usually don't
worry about too much. If it is a reasonable match, you can't really see it
from more than a foot away and gelcoat tends to yellow a little with time.
If the repair looks whiter than the rest of the boat give it a season. It'll
probably match itself in nicely. Polishing and waxing the area is, of
course, the final step. The wax itself will even yellow the new gelcoat
slightly.

Gelcoat is very forgiving stuff, so don't worry if it doesn't come out quite
right the first time. At worst, you will have a new gelcoat layer where
there was raw fiberglass and you can always go back to this area, touch it
up and practice the process over again, until you get it just the way you
want it to look. In the mean time, you are learning a skill that boat owners
pay dearly for, everyday. You will get a feel of how this best works for you
and you'll never worry about the odd nick your boat receives, 'cause you
know it can be fixed easily. The process does sound very involved on paper.
This is partially because I've tried to break it down into detailed steps
and partly due to my long windedness ;). However, it is really easy, once
you get the hang of how to do it.
Good luck on this project and if you get stuck at some point, or things
don't go as planned, do feel free to drop me a line, with questions. Hope
this has been helpful.